This proposal seeks to define the mechanisms by which films of pulmonary surfactant become capable of sustaining low surface tensions in the lungs. Adsorption of surfactant vesicles lowers surface tension from 70 mN/m for a clean air/water interface to ~25 mN/m, but multiple methods show that alveolar surface tensions reach values <5 mN/m. These lower values occur in dynamic lungs when the decreasing alveolar surface area during exhalation compresses the adsorbed films. To reach these low surface tensions, the films must avoid collapse of the film from the interface by flow into a three dimensional bulk phase. Two monomolecular films related to pulmonary surfactant can mimic the ability of the surfactant films to reach and sustain the very low surface tensions that occur in the alveolus. Dipalmitoyl phosphatidylcholine (DPPC), which is the most prevalent components of surfactant from most species, forms a highly ordered interfacial phase at physiological temperatures that resists collapse during compression and reaches very low surface tension. Disordered films containing either individual phospholipids or the complete set of surfactant constituents also become capable of sustaining very low surface tensions if compressed fast enough to outrun collapse and reach these low surface tensions. Although both the DPPC and supercompressed films can mimic the behavior of the alveolar film, mechanisms by which either structure could form are unknown. Processes that could produce the change in composition from an initial film containing all surfactant constituents to one that includes only DPPC is unclear. The supercompressed films also seem to require compressions faster than are necessary in the lungs for the films to reach low surface tensions. The proposed studies, which are largely based on these two films, consider specific mechanisms by which physiological processes could generate either film. The experiments test specific hypotheses concerning how the films become capable of sustaining low surface tensions. Our methods emphasize specific structural and functional changes predicted by different models, and specific dependence on composition. The models follow directly from results obtained during previous periods of this grant, and the experiments rely on methods established during those studies. The experiments should address a fundamental unanswered question of pulmonary physiology that has direct implications for the mechanisms of disease and for the development of new therapies. PUBLIC HEALTH RELEVANCE: Pulmonary surfactant forms films that coat the surface of a thin liquid layer that lines the small air sacks of the lungs, and minimizes their tendency to deflate. The mechanisms by which the surfactant films become exceptionally stable, and particularly able to prevent collapse of the air spaces, are unknown. The proposed studies seek to determine these mechanisms as a basis for understanding how pulmonary surfactant functions in the lungs, and how artificial therapeutic surfactants might be designed for the treatment of pulmonary diseases.